Generally, vehicle bodies passed through a welding stage are subjected to a coating process. For coating, first of all, cleaning and electrodepositing coating as base coating will be conducted. In this process, the electrodeposition coating, for example, is disclosed in Japanese Patent Application Laid-Open Publication No. 2002-097596 (JP 2002-097596 A). The technique disclosed in JP 2002-097596 A will be discussed with reference to FIG. 8 hereof.
As shown in FIG. 8, an immersion treatment apparatus 100 comprises a liquid storage tank 102 storing an electrodeposition liquid 101, and a vehicle body conveyance mechanism 105 for immersing vehicle bodies inclinedly into the liquid storage tank 102, conveying the vehicle bodies horizontally within the tank and pulling the vehicle bodies inclinedly out from the tank.
The vehicle body conveyance mechanism 105 comprises a downward-sloped guide rail 106, a horizontal guide rail 107 continuing from the downward-sloped guile rail 106, an upward-sloped guide rail 108 continuing from the horizontal guide rail 107, hangers 109 movably hung or suspended from the guide rails 106-108 for carrying vehicle bodies 103, and a power rail 111 for moving the hangers 109 from right to left as indicated by arrows in the FIG. 8.
Vehicle bodies 103 supported on the hangers 109 are guided by the downward-sloped guide rail 106 into the electrodeposition liquid 101. The vehicle bodies 103 are continuously conveyed by the horizontal guide rail 107 within the electrodeposition liquid 101 for achieving the required electrodeposition coating. Next, the vehicle bodies 103 are guided by the upward-sloped guide rail 108 out from the electrodeposition liquid 101.
By changing the electrodeposition liquid to a cleaning liquid, the vehicle bodies can be cleaned. Thus, the immersion treatment apparatus may be installed in the coating line, not only in that form having the electrodeposition tank but also in a form having the cleaning tank disposed forwardly (or rearwardly) of the electro-deposition tank.
Cleaning and coating vehicle bodies within a liquid is thus called immersion treatment.
In treating multiple vehicle bodies 103 continuously in the liquid storage tank 102, collision of adjacent vehicle bodies 103 should be avoided. To avoid such collision, it is desirable that the power rail 111 be moved at a constant speed.
It is known that if the moving speed is too high upon entry of the vehicle bodies into the liquid, the vehicle bodies 103 are liable to float. Thus, the moving speed should be set relatively low. If the moving speed is set low, the vehicle bodies are pulled out of the liquid at that low speed. It is desirable that foreign substances (dust and steel powders resulted from welding) remaining in the vehicle bodies 103 are removed together with the liquid as the vehicle bodies are pulled out of the liquid. However, if the moving speed is set low, only little foreign substances can be removed.
If chemical treatment or electrodeposition coating is conducted on steel plates with foreign substances adhered to surfaces of the steel plates, the quality of resultant chemical treatment film or electrodeposition coating is deteriorated significantly because such foreign substances are included in the chemical treatment film or electrodeposition coating.
Therefore, there is a demand for a technique to solve such a problem.